Make Tester extensible and framework-aware

[rubenvanassche]

Summary

Tester becomes an abstract base shared by framework adapters. SymfonyTester adds Symfony Console I/O on top, frameworks like spatie/laravel-flare extend it for framework-specific pre-checks and entry-point info.

What changed

• Tester is abstract; run() orchestrates pre-checks, sending and rendering. Write hooks (writeLine/writeNewline) are abstract and supplied by subclasses.
• New SymfonyTester wires Symfony Console (InputInterface/OutputInterface) and provides a default CLI entry point.
• New shared/FakeSymfonyTester.php for downstream packages that want to test their own subclass against the same harness.
• Style constants (STYLE_PLAIN/STYLE_INFO/STYLE_WARNING/STYLE_ERROR) replace the prior level-based naming.
• Pre-check now warns when logs would be sent with a non-DaemonSender.
• Test trace simulates a CLI command (Command span, derived from the entry point) instead of a web request.
• Test error report carries entry-point attributes and a Command span event.
• Test logs have varied per-level context to better exercise the pipeline.
• FlareProvider no longer registers Tester in the container (it's abstract now; subclasses are constructed by their owning packages).
• symfony/console added to require.

This PR is a prerequisite for the matching laravel-flare PR which moves TestCommand's remaining logic into a LaravelTester subclass.